Proton-exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells (or PEMFC) are a type of fuel cell developed for applications in transportation and also for portable applications. The principle of fuel cells was demonstrated experimentally in 1839 by the British electrochemist Sir William Grove. The first fuel cells of PEMFC type were developed in the United States in the 1960s by General Electric for space applications. Currently, this type of cell, which is designed to function at intermediate temperatures (40-120° C.), is developed internationally by the motor vehicle and portable electronics industries. However, despite undeniable environmental advantages, and superior energy yields, fuel cells are only just beginning to compete with internal combustion engines on account of the costs that are still high (raw materials, service lives).
The core of a fuel cell of PEMFC type is composed of a polymer electrolyte membrane, electrodes (anode and cathode, usually in the form of thin layers of platinum) and bipolar plates serving for gas diffusion.
Fuel cells functioning with proton-exchange polymer electrolyte membranes allow the conversion of the chemical energy of gases (H2/O2) into electrical energy with high energy yields and with no discharge of pollutant, according to the following equations:
Reaction at the cathode (site of oxygen reduction):½O2+2H++2e−→H2O
Reaction at the anode (site of hydrogen oxidation):H2→2H++2e−
Since the two electrodes are separated by the electrolyte (membrane), the fuel to be oxidized (hydrogen) is conveyed to the anode, and the cathode is fed with oxygen (or more simply with air, optionally enriched with oxygen). At the anode, the dihydrogen reacts and releases two electrons (oxidation) that feed an external electrical circuit connecting the anode and the cathode. At the cathode, the cathodic reduction of the oxygen takes place. The reagents are, in principle, introduced continuously into the device and the electromotive force of the cell is equal to the difference of the electrode potentials. A universally known overall reaction is thus obtained:H2+½O2→H2O
Water is thus produced by the normal functioning of the cell, which must be evacuated outside the membrane. Management of the water is crucial for the performance of the cell; care should be taken to ensure that the amount of water remains constantly at an optimum level, ensuring correct functioning of the cell. In particular, an excess of water leads to excessive swelling of the membrane, to congestion of the distribution channels or electrodes and to a negative effect on the access of the gases to the catalytic sites, whereas an insufficient amount of water leads to dryness of the membrane, which is harmful to its conductivity and to the yield of the cell.
The role of the membrane is thus to ensure the transportation of protons (H+) from the anode to the cathode and thus to allow the electrochemical reaction. However, it should not conduct electrons, which would create a short circuit in the fuel cell. The membrane should be resistant to the reductive environment at the anode and, at the same time, to an oxidative environment at the cathode, and should also prevent the mixing of the hydrogen contained at the anode with the oxygen contained at the cathode.
One of the first proton-transporting polymers that was used for producing such membranes, and which today remains the reference in this field, is Nafion®, a perfluorosulfonic polymer developed and perfected in 1968 by the American company Du Pont de Nemours. Historically, the NASA Gemini space programs of the 1960s used fuel cells comprising membranes of sulfonated polystyrene type, but these were very quickly supplanted by Nafion® membranes, which made it possible to improve the performance of PEMFCs. In chemical terms, it is an organic polymer, formed from a flexible fluorocarbon chain on which are randomly distributed ionic groups (Mauritz K. A. et al., Chem Rev., 2004, 104, 4535-4585). On the principle of Nafion®, other perfluorosulfonic commercial polymers exist, such as those sold under the trade names Aciplex® (Asahi Chemical Company, Japan) or Flemion® (Asahi Glass Company, Japan).
The membranes manufactured from these polymers are by nature very chemically, thermally and mechanically stable (flexibility). They have good electrochemical properties with high conductivity, of the order of 0.1 S·cm−1 at room temperature and at 100% relative humidity (according to the manufacturer's data for Nafion®). However, these membranes must operate at a temperature below 90° C. and must always remain saturated with water to allow efficient movement of the H+ ions. Specifically, the production of protons takes place mainly via a mechanism of Grotthus type, i.e. via the jumping of protons along the ionic and hydrophilic conduction pathways (Mauritz K. A. et al., 2004, mentioned above). Moreover, the synthesis of these membranes is long, difficult, or even dangerous taking into account the use of fluorine, which partly justifies their very high cost price. Furthermore, they are not entirely satisfactory as regards the problems associated with water management and temperature changes. Specifically, when the system undergoes numerous variations in the level of humidity, the appearance of successive cycles of swelling and restructuring of the membrane is noted, leading to substantial fatigue. Moreover, Nafion® is a polymer that quite naturally undergoes a glass transition (Tg=120° C.), which contributes toward its accelerated aging and to the appearance of structural reorganizations and mechanical weaknesses (failures), thus limiting its service life.
Other alternative types of polymers that may be used for the preparation of electrolyte membranes have also already been proposed. These are in particular sulfonated or doped thermostable polymers (sulfonated polyaryl ether ketones, polybenzimidazoles, polyaryl ether sulfones, etc.). These polymers lead to membranes that also have certain drawbacks, especially in terms of conductivity (performance), service life and water management.
Patent application US 2005/0 164 063 describes the synthesis of various solid compounds and electrolytes obtained from silsesquioxane-based precursors in which a siloxane function is attached to a phenylsulfonate group via a divalent group free of urea functions. Such structures, in which the divalent group linking the siloxane function to the phenylsulfonate is an alkyl or aryl radical, have the drawback of having poor conductivity (Electrochimica Acta, 2003, 48, 2181-2186).
This is why, to overcome the respective weaknesses of each of these systems, many studies for modification by incorporation of inorganic phases have been performed in recent years, and have led to an overall improvement in the properties of PEMFCs. This is reflected in particular in the water management and also in the behavior of the materials at high temperature (dehydration) and their long-term stability. These concepts became generalized with the appearance of hybrid membranes, which also make it possible to demonstrate the importance of the presence of a continuous inorganic network within the conductive electrolyte.
Rhodium-based monomer complexes obtained by reacting (p-aminophenyl)diphenylphosphine with 3-isocyanatopropyltriethoxysilane, with improved catalytic properties, are also known and used for sol-gel polymerizations (J. Organomet. Chem., 2002, 641, 165-172).
Films prepared by reacting an alkoxysilane with 4-[(4″-aminophenyl)sulfonyl]-4′-[N,N′-bis(2-hydroxyethyl)amino]azobenzene via sol-gel polymerization are also described in Chem. Mater., 1998, 10, 1642-1646, for their uses in the field of optics.
Unfortunately, at the present time, no membrane, irrespective of its nature, fully satisfies the rigorous requirements of PEMFC constructors and users. Although many operational technical devices using these electrochemical systems have appeared on the market, for instance the Genepac fuel cell arising from a partnership between PSA Peugeot Citroen and the Commissariat à l'Energie Atomique, and whose power may be up to 80 kW, technological obstacles remain to be overcome.
Firstly, in terms of water management: as has been seen previously, it is of paramount importance to manage the water produced during the functioning of the cell and its influence on the properties of the electrolyte membrane (especially the conductivity). This need to optimally control and manage the water transports taking place in a PEMFC (inlets, outlets, generation, and back-diffusion between cathode and anode) remains a major constraint that encourages the production of electrolytes that are less dependent on the relative humidity.
In terms of operating temperature also: fuel cells of Nafion® type can only work at maximum temperatures of 90° C. For higher temperatures, the membranes can no longer ensure suitable proton conductivity on account of their inability to retain water. Their yield decreases as a function of the drop in relative humidity combined with the increase in temperature. Now, the application of fuel cells to transportation vehicles requires the use of membranes that can function satisfactorily at temperatures above 90° C., in particular at temperatures between 120 and 150° C. Membranes of this type do not currently exist on the market.
Finally, in terms of manufacturing cost: to enable the mass development of this technology and the generalization of these power generators destined for a bright future, the problem remains of the cost of manufacture of the electrolyte membrane per se, but also the cost of manufacture of the fuel cell core (AME) associated with the use of platinum as a catalyst.